The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for imaging vascular calcifications or bony structures.
Vascular calcifications are a major component of atherosclerotic disease, and are often used for quantifying arterial disease as well as assessing risk for future cardiovascular morbidity and mortality. In clinical practice, computed tomography (“CT”) is used to visualize and quantify vascular calcifications. CT, however, involves the use of potentially harmful ionizing radiation and, thus, is not well suited for longitudinal and/or serial assessments of atherosclerotic disease, or for screening of the general population. MRI is a promising modality for visualizing and quantifying atherosclerosis without the safety concerns of ionizing radiation. Existing MRI techniques, however, cannot clearly and rapidly visualize arterial calcifications in patients with atherosclerotic disease.
A few approaches have been proposed for identifying vascular calcifications with MRI. One method referred to as “multi-contrast” (Cai J M et al. Circulation. 2002 Sep 10; 106(11):1368-73), includes the acquisition of a dark-blood T1-weighted fast spin echo (“FSE”) scan, a dark-blood T2-weighted FSE scan, a dark-blood spin-density weighted FSE scan, and a bright-blood time-of-flight (“TOF”) scan. This multi-contrast approach identifies vascular calcifications based on their dark appearance on all four acquisitions. Limitations of this approach include long scan times associated with multiple (typically four) scans, poor spatial resolution (because the FSE techniques are multi-slice 2D techniques and not 3D), and artifacts in the TOF imaging, such as arterial inhomogeneity and saturation. Furthermore, interpretation of multiple image sets is time consuming and can be significantly impaired due to patient motion in one or more of the scans.
Another approach for visualizing calcifications involves the use of 3D dark blood acquisitions (either FSE or gradient-echo), which provide images that depict the arterial wall. Although these techniques can visualize vascular calcifications based on their dark appearance in relation to the adjacent vascular wall, many other structures within the field of view appear dark, including the arterial lumen and perivascular fat. The presence of a considerable amount of dark-appearing perivascular fat renders vascular calcifications less conspicuous. With 3D dark blood techniques, superficial vascular calcifications are particularly difficult to discern because they are indistinguishable from the dark lumen. The offshoot technique of “gray-blood imaging” (Koktzoglou I, Magn Reson Med. 2013 July; 70(1):75-85), solves the poor contrast between superficial vascular calcifications and the vascular lumen, but does not solve the issue of poor contrast between perivascular fat.
In another recent approach, described by Q. Yang, et al., in “Imaging the Vessel Wall in Major Peripheral Arteries using Susceptibility Weighted Imaging,” J Magn Reson Imaging, 2009; 30:357-365, susceptibility-weighted imaging (“SWI”) was used in an effort to detect calcifications on the basis of their increased diamagnetic susceptibility. Although this technique shows some promise, it can be difficult to reliably obtain accurate phase maps, which are required for SWI, near peripheral vessels due to the presence of blood flow, fat-water interfaces, and bony structures that all produce additional phase shifts. An additional drawback of SWI for imaging calcifications is that it requires extensive offline image processing.
In light of the foregoing, there remains a need to provide systems and methods for accurately and reliably imaging vascular calcifications with MRI. It would therefore be desirable to provide systems and methods that can obtain images that conspicuously depict vascular calcifications without the drawbacks in scan time, spatial resolution, and artifacts associated with currently available techniques.